This invention relates to a method of determining the charge status of a storage battery, and to a method of determining the aging of a storage battery.
More accurate determinations are needed of when the low-charge status of a storage battery is actually reached. This low-charge status equates to a terminal voltage corresponding to a residual capacity of, for example, 10 to 20 percent of the rated capacity of the battery. With this residual charge it is still possible to operate the power-consuming device reliably. The battery voltage drops as its capacity (i.e., its charge) drops.
If the power-consuming device places a higher than normal load on the storage battery (i.e., if a greater amount of current flows through the storage battery), the storage battery's terminal voltage will drop at an accordingly higher rate that with a current of normal magnitude. Inside the storage battery itself a voltage drop occurs that is equal to the product of the internal resistance of the storage battery and the current. From this it results that a drop in the storage battery's terminal voltage equivalent to a drop in capacity of the storage battery can also be observed when the current increases. If, therefore, the terminal voltage is used as the sole criterion for determining the storage battery's capacity, an accordingly far advanced discharge status of the storage battery will be assumed to exist when lowering of the voltage is caused by a comparatively large current through the power-consuming device, even though its capacity is comparatively well above the low-charge point. In this case the storage battery still has a comparatively large capacity.
A prior art method for determining and displaying charge status is disclosed in DE-OS 41 31 981 A1 (U.S. Pat. No. 5,623,193), which describes how the operating time of an electrical appliance, particularly a shaving apparatus, is monitored with a counter. This prior art method features the activation of segments of a display in accordance with the operating time, indicating to the user the current charge status of the storage battery or batteries. If a voltage discriminator detects that the measured terminal voltage of the storage batteries has reached a predetermined lower threshold value, then the readout of the display is set to a condition indicative of a charge status that corresponds to the lower threshold value and a counter presetting a specified residual operating time of the electrical appliance is started. After this residual operating time has elapsed, the electrical appliance is switched off by appropriate activation of an electronic switching device. This switching off can be effected by instant switch-off or by a slow reduction of power, thus resulting in the case of a shaving apparatus in a slow reduction of speed.